The present disclosure relates to a reading module for reading image light resulting from reflection of light applied to a document, the reading module being for use in digital copiers, image scanners, and the like. The disclosure also relates to an image reading device and an image forming apparatus each including the reading module.
As the reading method for image reading devices which are to be mounted on a multifunction peripheral or the like using electrophotographic process, conventionally available are the CCD (Charge Coupled Device) method which uses charge coupled devices called CCD sensors, and the CIS method which uses photoelectric conversion devices called CMOS (Complementary MOS) sensors.
The CCD method is a method in which image reading is fulfilled by using plural plane mirrors and optical lenses in combination with an image sensor having a ⅕ to 1/9 size relative to a document size so as to form a scaled-down image. A large depth of field can be mentioned as a merit of the CCD method. The term ‘depth of field’ herein refers to a range over which it seems that a proper focusing has been obtained even though a subject (document in this case) has shifted in an optical-axis direction from an accurately-focused position. This means that a large depth of field allows a scarcely deteriorated image to be read and captured even though the document has shifted from a specified position.
On the other hand, a demerit of the CCD method could be that its optical path length (a traveling distance of light from subject to sensor) is quite long as much as 200 to 500 mm. In image reading devices, the light traveling direction is changed by using a plurality of plane mirrors in order to ensure the optical path length within a space of limited carriage. For this reason, parts count is increased, leading to a cost increase. With a lens used in the optical system, there occurs chromatic aberration due to differences in refractive index attributable to wavelength. Correcting this chromatic aberration necessitates a plurality of lenses. This use of plural lenses also causes a cost increase.
The CIS method is a reading method in which a plurality of erect-image, actual-size rod lenses are arrayed so as to make an image formed on an image sensor similar in size to the document. It can be mentioned as a merit of the CIS method that the optical path length is relatively short such as 10 to 20 mm, leading to a small scale, as compared with the CCD method. Also, since rod lenses alone are used to fulfill image formation, mirrors that would be necessitated for the CCD method are no longer necessary. As a result, the scanner unit on which the CIS sensor is mounted can be provided a thinner type one, hence a simple structure and a resultant lower cost as additional merits. On the other hand, the CIS method, because of its very small depth of field, undergoes significant influences of blurs due to image smears caused by differences in scale factor among individual lenses when the document has shifted in the optical-axis direction from the specified position. As a result, the CIS method has a demerit of incapability of uniformly reading a book document or a depressed-and-projected document.
In recent years, there has been proposed a method in which image reading is fulfilled by using a reflecting mirror array for the imaging optical system, unlike the above-described CCD method and CIS method. In this method, reflecting mirrors are disposed in array so as to form scaled-down, inverted images of the document for their corresponding reading areas, respectively. However, one area is read and imaged by one optical system, unlike the CIS method using a rod lens array. Further, by virtue of adopting a telecentric system for the imaging system, there occur no image smears due to superimposition of images of different scale factors during process of reading the document for individual plural areas. Thus, image blurs are suppressed, and a compound-eye reading method is established.
Furthermore, in this method, in which only mirrors are used in the optical system, chromatic aberration never occurs unlike cases where lenses are used in the optical system. Accordingly, there is no need for correction as to chromatic aberration, allowing the count of elements constituting the optical system to be reduced.